Structural coloration caused by the interaction of light with nanoscale periodic structures, so-called photonic crystals, has attracted considerable attention in a variety of research areas. The potential applications range from decorations to display technologies. Structural color has many characteristics that differ from those of pigments or dyes. The unique colors originating from the physical structures are usually iridescent and metallic, and cannot be obtained by chemical dyes or pigments. Moreover, structural color is brighter, more deeply saturated, free from photo-bleaching, and longer lasting, unlike traditional pigments or dyes.
In the biological world, the colorful feathers of many birds (i.e. peacock), the wings of various butterflies and the elytra of many beetles are good examples of structural coloration. Apart from the aforementioned properties, natural structural colors produced by these animals also give rise to additional optical properties. For instance, the brown barbules of peacock feathers adopt mixed structural coloration; the green wing scales of Papilio Blumei produce polarization effect and bi-structural-color reflection. In particular, bi-structural-color reflection is responsible for various crucial functions in the animal kingdom. The widespread evidence indicates that animal photoreceptors can distinguish different parts of the spectrum, even also sensitive to ultraviolet (UV) light. It follows that the bi-structural-color reflection of certain butterflies cover both the visible and UV ranges, which may serve as communication and mating signals. The construction of an effective and simple way to produce multiple-structural-color reflection covering UV, visible and infrared (IR) ranges simultaneously may point to a new direction in multi-functional photonic materials engineering.
Bi-structural-color reflection can be created by different manners in the biological world. Under UV-visible light, the blue wing scales of the butterfly Papilio Ulysses display two distinct reflection peaks. Specifically, the concavities of a multi-layer structure and the ridges of a 2D photonic-crystal structure occurring at the wing scales produce green and UV reflections, respectively. On the other hand, the green wing scales of the butterfly Papilio Blumei produce the reflections of two different visible colors. The yellow and blue reflections created by the flat bottoms and inclined sides of the concavities, respectively, give rise to a color mixing, leading to a green color as perceived by human eyes. The aforementioned bi-structural-color reflection produced by the two Papilio butterflies are of different structural origins: in P. Ulysses it is produced by two different types of ordered structures of the wing scales, while in P. Blumei it is produced by the different illumination angles from different parts of the wing scales, which share the same ordered nano-structure.